1. Field of the Invention
The present invention relates to a coordinate input apparatus and, more particularly, to a coordinate input apparatus in which a vibration which was input by a vibration pen is detected by a plurality of vibration sensors attached to a vibration propagating plate and the coordinates of the vibration pen on the vibration propagating plate are detected.
2. Related Background Art
A coordinate input apparatus in which the lateral wave component of the plate wave elastic wave which was input onto a vibration propagating plate by a vibration input pen is detected and the coordinates of the vibration input point are calculated has been disclosed in a commonly-assigned U.S. Pat. No. 4,931,965 filed on Jun. 30, 1989, and issued Jun. 5, 1990 by Kaneko et al.
In the above apparatus, the accurate distance from the input point is obtained from the relation between the vibration propagation time based on the group velocity (group envelope signal) and the vibration propagation time based on the phase velocity (phase signal). In the apparatus, the detection timing of the vibration propagation time due to the envelope signal is used as a reference and the vibration propagation time due to the phase signal is detected.
That is, since there is a predetermined relation between the group envelope signal and the phase signal in accordance with the vibration propagation distance, a predetermined characteristic point (for instance, the peak point) is first detected with respect to the envelope signal and is compared with a predetermined threshold value with respect to the phase signal by using the detected timing as a reference. For instance, in the portion in which the phase signal first exceeds the predetermined threshold value, a gate signal of a predetermined time width is given. Further, the first leading point (zero-cross point) of the phase signal in the gate signal is detected. Due to this, the vibration propagation time can be stably detected in terms of the detecting system.
However, since a time lag generally occurs in the detection of the characteristic point of the group envelope signal, if the detection timing is used as a reference, the unstable signal portion of the rear portion of the phase signal is detected with respect to the phase signal. That is, FIG. 14 is a diagram showing an example of the signal detected by the vibration sensor. In the diagram, a central portion a is constructed by a stable wavelength (frequency) but a rear portion b is constructed by an unstable wavelength (frequency) which is being attenuated and is oscillating.
It is also considered a method whereby a delay circuit is provided for a phase signal circuit in order to match with the delay time in the detection of the characteristic point of the envelope signal as mentioned above. However, this results in increase in the cost of the apparatus.
On the other hand, there is also a case where the vibration sensor receives external disturbance noises or where noises of another circuit, for instance, a digital signal processing system or the like are mixed into the detection signal of the vibration sensor. Therefore, if the gate signal is given on the basis of such an unstable detection signal, the determination of the input coordinates also becomes unstable.